This invention relates generally to ultrasonic spray nozzles and in particular to an ultrasonic spray nozzle and method wherein drive energy to the nozzle is frequency modulated and wherein auxiliary fluid-flow ports are provided in the nozzle tip such that a well defined spray pattern is produced.
Ultrasonic nozzles which operate at a single drive frequency are well known and offer numerous advantages over conventional hydraulic and pneumatic spray nozzles. Typically, such ultrasonic nozzles provide reduced spray velocities, infinitely variable control of fluid spray rates and significantly reduced operating power consumption.
In contrast to conventional spraying mechanisms which rely on relatively high hydraulic pressures or high velocity gas streams for atomization of sprayed liquid media, ultrasonic nozzles utilize the ultrasonic mechanical vibrations of a piezoelectric transducer to vibrate an atomizing surface and thereby atomize a fluid disposed thereon. The absence of such pressures and gas streams results in the development of a droplet fog wherein the average velocity of individual droplets is very low compared to those produced by other atomizing techniques. Although a low average droplet velocity is of great benefit in that overspray and excess fluid delivery are both reduced, spray patterns made up of such low velocity droplets are often poorly defined. Accordingly, definite measures must be taken whenever the spray pattern shape provided by an ultrasonic nozzle is of importance.
One well-known technique for controlling the spray pattern of an ultrasonic nozzle involved entraining the spray droplets in a moving air stream and then shaping the air stream to provide the desired spray pattern. While this technique was effective, it had the disadvantage of requiring often complex, bulky, and expensive air blowers and related equipment.
Another well-known spray pattern control technique involved the use of a shaped atomizing surface in the construction of the ultrasonic nozzle. This technique was based on the principle that the individual droplets, produced when a uniform liquid film is atomized by an ultrasonically vibrating surface, will be thrown off in a perpendicular direction relative to the surface. Accordingly, the initial shape of the spray pattern produced by such an ultrasonic nozzle should, in theory, be related to the shape of the generating atomizing surface.
Although a properly shaped atomizing surface was found to advantageously influence the shape of the spray pattern it produced, it was found, in practice, that the pattern nevertheless tended to waver in space and become diffuse, particularly so in the region located more than a few inches from the atomizing surface. Such diffusion and wavering destroyed the definition of the spray pattern and resulted in areas of greater and lesser droplet concentrations along the spray pattern front. This, in turn, adversely affected the uniformity with which sprayed material could be deposited onto a substrate and was of particular significance in various processes, such as in the manufacture of pharmaceuticals, wherein it was desired to precisely deliver a known and minute quantity of material to a substrate so as to achieve a uniform concentration of the material therein.
Another difficulty associated with ultrasonic nozzles was the need to provide an independent drive source for each nozzle when two or more nozzles were to be operated simultaneously. Though the mechanical construction and operation of ultrasonic nozzles was greatly simplified over that of conventional hydraulic and pneumatic spraying mechanisms, effective ultrasonic nozzle operation was a result of careful design which sought to maximize the amplitude of the mechanical vibrations appearing on the nozzle atomizing surface. This was achieved by relating various nozzle dimensions to the vibrational wavelength provided when the nozzle was operated at a particular frequency. When properly designed, the natural resonant frequency of an ultrasonic nozzle would match that of an applied electrical drive potential and, ideally, would maximize the vibrational amplitude of the atomizing surface.
Although careful design and construction would result in a close match between the actual nozzle resonant frequency and the nominal design frequency, practical manufacturing tolerances, would, in most cases, reduce the probability of an exact correspondence between these frequencies. As a result, each nozzle, even though designed for operation at the same nominal operating frequency, would nevertheless have a particular, and in all likelihood, unique, operating frequency at which optimum performance was obtained. Accordingly, in use, the actual frequency of the nozzle drive signal was carefully adjusted to match the natural nozzle resonant frequency in order to obtain best results. This generally required that each nozzle of a multi-nozzle system be operated from its own dedicated energy source since the effort required to provide two or more perfectly matched nozzles far exceeded the savings to be realized in utilizing a single drive energy source.
The present invention is directed to an ultrasonic spray nozzle system and method wherein a parameter of the ultrasonic energy applied to the nozzle is varied with respect to time so as to result in a periodic increase and decrease in the vibrational amplitude of the nozzle's atomizing surface. This permits fluid to more uniformly cover the atomizing surface during periods of low vibrational amplitude and to thereafter be atomized into a well defined spray pattern during periods of increased vibrational amplitude. To further enhance the definition of the resulting spray pattern, the nozzle can be provided with one or more auxiliary fluid-flow ports which function to evenly distribute the fluid over the atomizing surface during periods of reduced vibrational amplitude.
In one principal aspect of the present invention, an ultrasonic nozzle includes a piezoelectric transducer which expands and contracts in response to an applied periodic electrical potential. The expansion and contraction of the piezoelectric transducer develops mechanical vibrations which appear on an atomizing surface formed on a portion of the nozzle. A parameter of the applied periodic electrical potential is modulated with time such that the vibrational amplitude of the atomizing surface is alternately increased and decreased.
In another principal aspect of the present invention, an ultrasonic nozzle, having an atomizing surface, includes a fluid passage which opens through the atomizing surface at a first location thereon. One or more auxiliary passages, which communicate with the main fluid passage, open through the atomizing surface at remote locations and function to communicate fluid to the atomizing surface such that the fluid is evenly distributed thereon.
In still another principal aspect of the present invention, the ultrasonic nozzle has a characteristic resonant frequency and the frequency of the applied drive energy is periodically varied from below to above the resonant frequency of the nozzle.
In still another principal aspect of the present invention, two or more ultrasonic nozzles are operated from a single source of drive energy. The drive energy frequency is modulated so as to periodically sweep through the resonant frequency of each nozzle. This assures that resonance is independently achieved in each nozzle over at least a portion of each frequency sweep cycle.
These and other objects, features, and advantages of the present invention will be clearly understood through consideration of the following detailed description.